POWDER/BULK SOLIDS

Technical Feature
In-Process Weigh Scale Yields Efficiency, Safety, Information Gains

Background

Two global leaders in the production of plastic products wished to increase the efficiency and safety of their operations in the plastic regrind area, and to increase the amount and timeliness of information available to them on the overall process yield. The manufacturers are suppliers of some 14,000 consumer, institutional and industrial products, including trash bags, food bags and wrap, poly-sheeting and specialty products.

For the past 5 years, these companies have selected CompuWeigh Corporation of Woodbury, Connecticut, USA as the exclusive supplier of all bulk scales for their in-process weighing applications.

Description of Problem
The companies experienced similar difficulties in efficiency, safety, and information collection.

Efficiency
Efficiency suffered, because at each facility a dedicated operator was required to:

• Monitor the filling of drums (approximately 50 Kg. each) from the single-screw regrind extruder. These drums would fill at varying rates, requiring from 5 to approximately 20 minutes to fill, the operator was therefore required to be on-station and attentive at all times. Once a drum filled with extrudate, the operator would:
• Manually lift the drums onto a forklift truck
• Drive the forklift truck carrying the filled drum to a larger “gaylord” of approximately 500 Kg. capacity, and dump the drum. When the gaylord was filled to capacity, the operator would:
• Move the filled gaylords by forklift to a platform scale for weighing, then finally
• Move the filled gaylords yet again from the platform scale to large collection bins where the polymer was stored for later processing.

The expense of the dedicated operator to move the filled drums, and the potential for injury to the operator were considered unacceptably high.

Safety
Safety was a problem, as forklift trucks were required to move the drums and gaylords over long distances. Since the drums would sometimes fill as frequently as every five minutes, forklift traffic was found to be a significant hazard. Also, the potential for injury to the operator was high, as this labor-intensive procedure would sometimes cause back injuries as the operators manipulated the filled drums on and off the forklifts.

Information

Finally, because the forklift operators were forced to wait for a full gaylord before moving and weighing, yield information was available only when the filled gaylord was moved to the platform scale. Since the operators had to wait at least 45 minutes, and sometimes several hours, between weighing the filled gaylords, the lag in yield information feedback was unacceptable.
In addition to improvements to the procedures described above, the companies were also searching for a simple solution to monitor inventory levels without stopping operation, and without relying on bin level controls, which monitor inventory by level, not by weigh. If a product could be found which addressed all these needs at once, so much the better.

Search for a Solution

Of prime concern was the desire to reduce the amount of operator involvement and to increase the overall safety of the process by eliminating the need for filled drums altogether. Of secondary concern was monitoring inventory levels by weight (to calculate yields) in “real-time”.

The companies researched a number of possible solutions to the weighing problem including installation of load cells on collection bins, drum scales, forklift scales, and platform scales. These approaches were considered and rejected for one or more of the following reasons:

• High purchase price or installation cost
• Continued inefficiency
• Unacceptable safety
• Unacceptable accuracy
• Inventory not well tracked
• High ongoing maintenance costs

The Solution

The Process Hopper Scale and Scale Controller from CompuWeigh was found to offer the best combination of high accuracy (0.1% by weight), short cycle time, high throughput, low price, easy installation, near-zero maintenance requirements, and immediate access to data.

CompuWeigh‘s Process Hopper Scales are low-cost, self-contained scales designed to handle a range of dry bulk-materials. With short cycle times, the scale allows high throughput (from 0.5 – 90 TPH) while maintaining 0.1% accuracy. Gates and weigh hoppers are fabricated of stainless steel. The stand-alone scale is shipped fully assembled, wired, piped and pre-tested. Installation at the plant site involves connecting 115/230 VAC power and a 90PSI (630 KPa) air supply. The low-profile design (weigh hopper height is 2.5 ft. (0.7 m) – 3.5 ft (1.0 m)) and small footprint allows easy installation, typically accomplished in one 8-hour shift.

Results

The scales have been in place for as long as 5 years, operating from 16 to 24 hours per day. Both producers are as well satisfied with the improvements in efficiency and labor costs, and improved safety. Back injures and forklift traffic have been cut dramatically, as there is no longer a need to roll, lift, or transport filled drums – they’ve been eliminated from the process flow. The producers estimate savings in excess of US $20,000 per scale per year. As a side benefit, they are now able to monitor regrind inventory on an ongoing basis. One of the producers is also discussing complete facility automation with CompuWeigh, including inventory, temperature level monitoring, gate proportioning and man-machine interface.

CompuWeigh Corporation is the largest producer of bulk weighing scale controllers in the United States, manufacturing “legal-for-trade” computerized bulk weighing equipment since 1978. Accuracy of 0.1% by weight is standard. Scale fabrication is available, with capacities ranging from 0.5 tonnes/hour to 90 tonnes/hour for in-process applications to over 3000 tonnes/hour for receiving and load-out applications of bulk and powdered materials including grains, plastics, flour, coffee beans, edible oils, rice, chemicals and others.